Based on evidence gathered by these and other investigators, the applicants hypothesize that a single virus strain, rather than multiple variants, are transmitted during initial infection. They suggest that this single strain then replicates to high titer and can lead to the high level viremia detected in some acute infections. Virus-specific immunity is induced, which then controls the viremia. This proposal seeks to address these hypotheses by characterizing the following virologic and immunologic parameters during acute seroconversion: (1) the genotype and phenotype of transmitted viruses, (2) the HIV-1-specific cytotoxic T lymphocyte response, and (3) the HIV-1-specific antibody response. These studies of the interaction between HIV-1 and the immune system during primary infection should provide for a definition of the immediate events surrounding transmission and seroconversion, and they may have important implications for vaccine design and our understanding of viral pathogenesis.